Voltage-dependent calcium channels are transmembrane multisubunit proteins that control inflow of extracellular calcium ions into cells. The voltage-dependent calcium channels are further classified into various types in mammalian cells. Major types of the voltage-dependent calcium channels include L-type, T-type, N-type, P/Q type, and R-type calcium channels, which play respective roles in various tissues including skeletal muscles, cardiac muscles, lungs, smooth muscles, and brain. Among these types, the “T-type” (or “low-voltage activated-type”) calcium channel is named after its characteristic that has a shorter (T=transient) opening time than the L-type calcium channel that has a longer (L=long-lasting) opening time [Non-Patent Document 1].
The T-type calcium channels have channel characteristics, which are known to be a factor to open the L-type calcium channels and a factor to fluctuate the action potential of sodium channels. Here, hyperexcitability of nerves due to an abnormality (abnormal firing) in fluctuations of the action potential of the sodium channels is believed to be a pathogenesis of neuropathic pains. The T-type calcium channels are supposed to relate to the abnormal firing, and blocking of the T-type calcium channels are believed to suppress the abnormal firing itself and to suppress pains [Non-Patent Document 2].
More specifically, the T-type calcium channels identified in various mammals including humans include three subtypes, α1G (Cav3.1), α1H (Cav3.2), and α1I (Cav3.3). Among the three subtypes of the T-type calcium channels, α1H is expressed in the dorsal root ganglion (DRG) and the dorsal horn of the spinal cord, which relate to pain transmission [Non-Patent Document 2, Non-Patent Document 12]. In studies using all knockout mice, analgesic action has been disclosed in acute pain models (tail clip, tail flip, and hot plate tests), inflammatory pain models (capsaicin and formalin-induced tests), and visceral pain models (acetic acid and magnesium sulfate inductions). During the tests, no abnormality was observed in general behavior [Non-Patent Document 3].
Analgesic action has also been identified in neuropathic pain model rats (CCD) to which an antisense gene of α1H is administered to suppress the expression of α1H in the spinal cord [Non-Patent Document 4]. In addition, in the case of suppressing the expression in the DRG, analgesic action has been identified in neuropathic pain model rats (CCI) [Non-Patent Document 5].
As for the action on pains associated with diabetic neuropathy, in the DRG of pain model rats having diabetic neuropathy prepared by administration of streptozotocin, an increase in gene expression of α1H [Non-Patent Document 6] and an increase in T-type calcium channel current [Non-Patent Document 7] have been disclosed, and the pain suppressive action has also been identified by the intrathecal administration of an antisense gene of α1H to the pain model rats [Non-Patent Document 8]. It has been disclosed that the onset of pain has been completely suppressed in α1H knockout mice to which streptozotocin has been administered, and the expression of all in the DRG has increased and the administration of a T-type calcium channel inhibitor has provided an analgesic action in ob/ob mice as diabetic model mice [Non-Patent Document 9]. From these findings, compounds having the inhibitory activity on the T-type calcium channel should be used as a therapeutic agent for pain.
The T-type calcium channels is considered to relate to pains such as neuropathic pain, inflammatory pain, and cancer pain, as well as pathology of various diseases and disorders including epilepsy, essential tremor, schizophrenia, Parkinson's disease, depression, anxiety, sleep disorder, sleep disturbance, mental illness, schizophrenia, cardiac arrhythmia, hypertension, pain, cancer, diabetes, overactive bladder, chronic kidney disease, sterility, and sexual dysfunction [Non-Patent Document 2, Non-Patent Document 3, Non-Patent Document 10, Non-Patent Document 11, Non-Patent Document 13, Non-Patent Document 14].
Treatment methods for such diseases involve many problems, and thus there is a demand for novel pharmaceutical products. Although some compounds having the T-type calcium channel inhibitory activity have been disclosed (for example, see Patent Documents 1 to 3), there is a demand for development of new medicinal agents.